Thermal barrier coating systems of various types are well known in the gas turbine engine industry as one means for protecting nickel-based and cobalt-based superalloy components, such as turbine blades and vanes, from oxidation and corrosion during engine operation.
One type of thermal barrier coating system involves depositing on the superalloy component (substrate) to be protected an MCrAlY metal alloy overlay where M is iron, nickel, cobalt, or a combination thereof, oxidizing the metal alloy overlay to form an alumina layer in-situ on the bondcoat, and then depositing a ceramic thermal barrier layer having columnar morphology on the alumina layer. Such a thermal barrier coating is described in U.S. Pat. Nos. 4,321,310 and 4,321,311.
Another type of thermal barrier coating system exemplified by U.S. Pat. No. 5,238,752 involves forming on the superalloy component (substrate) to be protected a high aluminum, atomically ordered intermetallic compound as a bondcoat. The intermetallic compound comprises, for example, equiatomic nickel aluminide (NiAl) having an Al content of 31.5% by weight or platinum modified nickel aluminide known commercially as Chromalloy RT-22 having a high aluminum intermetallic NiAl Al matrix and includng PtAl.sub.2 phases in the coating microstructure. The intermetallic compound bondcoat is oxidized to form a thermally grown alumina layer in-situ thereon, and then a ceramic thermal barrier layer having columnar or other morphology is deposited on the alumina layer.
Still another type of thermal barrier coating system exemplified by U.S. Pat. Nos. 4,880,614 and 5,015,502 involves forming on the superalloy component (substrate) to be protected a metallic bondcoat which may comprise an MCrAlY metal alloy overlay or a diffusion aluminide layer predominantly composed of aluminum intermetallic (e.g. NiAl, CoAl, and (Ni/Co)Al phases) which may be modified with Pt, Si, Hf, Cr, Mn, Ni, Co, Rh, Ta, Nb, and/or particulates, chemical vapor depositing (CVD) a high purity alpha alumina layer on the metallic bondcoat, and depositing a ceramic thermal barrier layer on the CVD alpha alumina layer.
In the manufacture of thermal barrier coating systems, the ceramic thermal barrier material, such as yttria stalized zirconia, has been applied to the bondcoat by plasma spraying wherein coating adherence is promoted by the roughness of the bondcoat. Controlled porosity and microcracking within the ceramic thermal barrier layer accommodates strain developed due to the differences in thermal expansion coefficients between the ceramic and the substrate superalloy. Alternately, ceramic thermal barrier material has been applied to the bondcoat by physical vapor deposition (PVD), such as sputtering and electron beam evaporation, under conditions to produce a columnar morphology (i.e. independent ceramic columns) in the ceramic thermal barrier layer. This columnar morphology organizes the coating porosity between the columns to accommodate strain from thermal expansion mismatch between the substrate and ceramic thermal barrier layer.
An object of the present invention is to provide an improved thermal barrier coating system for use on gas turbine engine and other superalloy components or articles operating at elevated temperatures where oxidation and corrosion protection is needed.
Another object of the present invention is to provide a thermal barrier coating system by an improved method which results in advantages in the manufacture of thermal barrier coating systems.